Musculoskeletal allografts produced by tissue banks from human donor tissues are used in a variety of orthopedic reconstructive surgeries. Over 1.5 million musculoskeletal allografts are used in the United States each year and this count is increasing steadily (1). Annual demand specifically for large structural cortical bone allografts in Canada is estimated at 16,000 grafts per year (2). An extrapolated estimate for the US is 160,000-320,000 per year (3). Large bone allografts are effectively devitalized bone-based implants which do not repair like living bone or autografts. Therefore, a large structural bone allograft must be able to fulfill its role for the long-term while resisting approximately a million loading cycles a year. The primary concern is donor-to-patient disease transmission. Terminal sterilization by γ-irradiation is used to sterilize allografts, eliminating bacteria, fungi and viruses and therefore greatly lowering infection risk (4).
γ-irradiation causes significant degradation of the mechanical performance of bone. Fracture rates for non-irradiated structural bone allografts range from 18% (5;6) to 42% (>26 months post implantation) (7). Irradiated structural allografts fracture at twice the rate of fresh allografts (6). Irradiation results in a safer product in terms of risk of infection but at the expense of graft quality and a heightened risk of fracture and revision surgery. An effective sterilization method which uses irradiation but maintains mechanical performance is required.
Bone is a biological composite tissue made of a stiff and brittle mineral phase toughened by collagen. The collagenous phase inhibits crack initiation and propagation. Irradiation of bone collagen results in highly fractured peptide chains and loss of native molecular structure (8). These changes result in severely reduced fracture toughness (9;10) and fatigue life (8;11). Fracture surfaces from irradiated bone (36 kGy on dry ice) are relatively flat and missing the fiber pullout/bridging and tortuous crack diversions typically seen in native bone (8;10;11), indicating a more brittle failure mode and correlating with highly fractured collagen content. Irradiation reduces toughness in tension by 86.4% and reduces low stress fatigue life to 1/10th of normal and high stress fatigue life to 1/200th of normal (11).